The present invention relates generally to voltage reference circuits, and more specifically to bandgap voltage reference circuits.
Integrated circuits, and other electronic circuits, often require operating voltages that are stable over process, voltage, and temperature variations. One type of circuit that is commonly used to provide stable voltages is the bandgap voltage reference circuit. A bandgap voltage reference circuit takes advantage of the characteristics of the bandgap energy of a semiconductor material (e.g., silicon) to provide a stable reference voltage. At a temperature of absolute zero (i.e., zero Kelvin), the bandgap energy of a semiconductor material is typically a physical constant. As the temperature of the semiconductor material rises from absolute zero, the bandgap energy of the material decreases (i.e., a negative temperature coefficient is displayed).
The voltage across a forward biased PN junction (i.e., the junction between a positive (P) doped portion and a negative (N) doped portion of a semiconductor material) is an accurate indicator of the bandgap energy of a material. For this reason, the voltage across a forward biased PN junction will decrease as the temperature of the semiconductor material is raised. The rate at which the voltage decreases depends upon the junction (cross-sectional) area of the particular PN junction (as well as the semiconductor material being used). Therefore, the voltages across two forward biased PN junctions having different cross-sectional areas (but using the same semiconductor material) will vary at different rates with temperature, but each of these voltages can be traced back to the same bandgap voltage constant at absolute zero. The conventional bandgap voltage reference circuit utilizes the voltage relationships between two forward biased PN junctions having different cross-sectional areas to achieve a relatively temperature insensitive output voltage.
In a conventional bandgap voltage reference circuit, a feedback loop is used in conjunction with a differential amplifier to generate a reference voltage. The feedback loop maintains the two input nodes of the differential amplifier at approximately the same potential in the steady-state. A first input node (e.g., the non-inverting input node) of the differential amplifier is coupled to a reference potential through a first PN junction (e.g., a diode or transistor). A second input node (e.g., the inverting input node) of the differential amplifier is coupled to the reference potential through a resistor and a second PN junction that has a different cross-sectional area (typically larger) than the first PN junction. Substantially equal currents are forced through the first and second PN junctions during circuit operation. By carefully selecting circuit component values for the bandgap voltage reference circuit, a system can be achieved that balances the negative temperature coefficient associated with one of the PN junctions with a positive temperature coefficient associated with the difference in the PN junctions to generate a relatively temperature insensitive output voltage. For further discussion of bandgap voltage references, see H Banba, xe2x80x9cA CMOS Bandgap Reference Circuit with Sub-1V Operation,xe2x80x9d IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999.
Ideally, the gain of the differential amplifier is very high. In general, higher gain in the differential amplifier enables more voltage insensitivity to temperature. Many differential amplifiers have gain fluctuations as a function of common-mode input voltage. The steady-state common-mode voltage values at the input to the differential amplifier are a function of many variables, including the non-linear characteristics of the diodes or transistors used. The steady-state common-mode voltage values define an xe2x80x9coperating pointxe2x80x9d of the bandgap voltage reference. By maintaining the operating point in a region of high gain, the bandgap voltage reference can provide a more stable output voltage.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method and apparatus to modify the operating point of bandgap voltage references.